Active metal fuel cells

ABSTRACT

Active metal fuel cells are provided. An active metal fuel cell has a renewable active metal (e.g., lithium) anode and a cathode structure that includes an electronically conductive component (e.g., a porous metal or alloy), an ionically conductive component (e.g., an electrolyte), and a fluid oxidant (e.g., air, water or a peroxide or other aqueous solution). The pairing of an active metal anode with a cathode oxidant in a fuel cell is enabled by an ionically conductive protective membrane on the surface of the anode facing the cathode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/334,116 filed Dec. 12, 2008, titled ACTIVE METAL FUEL CELLS, now U.S.Pat. No. 7,781,108; which is a continuation of U.S. patent applicationSer. No. 10/825,587 filed Apr. 14, 2004, titled ACTIVE METAL FUEL CELLS,now issued as U.S. Pat. No. 7,491,458; which claims priority to U.S.Provisional Patent Application No. 60/529,825 filed Dec. 15, 2003,titled ACTIVE METAL FUEL CELLS, and to U.S. Provisional PatentApplication No. 60/518,948 filed Nov. 10, 2003, titled BI-FUNCTIONALLYCOMPATIBLE IONICALLY CONDUCTIVE COMPOSITES FOR ISOLATION OF ACTIVE METALELECTRODES IN A VARIETY OF ELECTROCHEMICAL CELLS AND SYSTEMS; thedisclosures of which are incorporated herein by reference in theirentirety and for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to active metal electrochemicaldevices.

More particularly, this invention relates to active metal (e.g.,lithium) fuel cells made possible by active metal electrode structureshaving ionically conductive membranes for protection of the active metalfrom deleterious reaction with air, moisture and other fuel cellcomponents, methods for their fabrication and applications for theiruse.

2. Description of Related Art

In recent years, much attention has been given to hydrogen and/or fossilfuel based fuel cells. A fuel cell is an electrochemical device thatcontinuously changes the chemical energy of a fuel (e.g., hydrogen) andan oxidant (e.g., oxygen in air or water) directly to electrical energy,without combustion. Fuel atoms give up their electrons in the process.Like a battery a fuel cell has electrodes and electrolyte. However,while a battery stores energy, a fuel cell generates it from fuel andoxidant supplied to the electrodes during operation. In a hydrogen fuelcell, oxygen is typically supplied to the oxygen electrode (cathode;electrode to which cations migrate) from ambient air, and the hydrogenfuel is supplied to the fuel electrode (anode) either from a pressurizedcylinder or from a metal hydride forming alloy. Fossil fuel based fuelcell systems Fossil fuel based fuel cell systems extract the requiredhydrogen from hydrocarbons, such as methane or methanol.

Active metals are highly reactive in ambient conditions and can benefitfrom a barrier layer when used as electrodes. They are generally alkalimetals such (e.g., lithium, sodium or potassium), alkaline earth metals(e.g., calcium or magnesium), and/or certain transitional metals (e.g.,zinc), and/or alloys of two or more of these. The following activemetals may be used: alkali metals (e.g., Li, Na, K), alkaline earthmetals (e.g., Ca, Mg, Ba), or binary or ternary alkali metal alloys withCa, Mg, Sn, Ag, Zn, Bi, Al, Cd, Ga, In. Preferred alloys include lithiumaluminum alloys, lithium silicon alloys, lithium tin alloys, lithiumsilver alloys, and sodium lead alloys (e.g., Na₄Pb). A preferred activemetal electrode is composed of lithium.

The low equivalent weight of alkali metals, such as lithium, render themparticularly attractive as electrode materials. Lithium provides greaterenergy per volume than the traditional hydrogen fuel or fossil fuel fuelcell standards. However, it has not previously been possible to leveragethe advantages of lithium and other alkali or other active metals infuel cells. Previously, there was no way to isolate the highly reactiveanode alkali metal fuel from the cathode oxidant while maintaining apath for the alkali metal ions.

SUMMARY OF THE INVENTION

The present invention relates generally to active metal electrochemicaldevices. More particularly, this invention relates to active metal fuelcells.

The present invention provides an active metal fuel cell. The fuel cellhas a renewable active metal (e.g., lithium) anode and a cathodestructure that includes an electronically conductive component (e.g., aporous metal or alloy), an ionically conductive component (e.g., anelectrolyte), and a fluid oxidant (e.g., air, water or a peroxide orother aqueous solution). The pairing of an active metal anode with acathode oxidant in a fuel cell is enabled by an ionically conductiveprotective membrane on the surface of the anode facing the cathode.

In one aspect, the invention pertains to a fuel cell. The fuel cellincludes a renewable active metal anode and a cathode structure thatincludes an electronically conductive component, an ionically conductivecomponent, and a fluid oxidant. An ionically conductive protectivemembrane is provided on the surface of the anode facing the cathode. Themembrane is composed of one or more materials configured to provide afirst surface chemically compatible with the active metal of the anodein contact with the anode, and a second surface substantially imperviousto and chemically compatible with the cathode structure and in contactwith the cathode structure.

The active metal anode is renewable in that it is configured forreplacement or supplementation of the active metal to provide a fuelsupply for continuous operation of the fuel cell for as long as desired.It may be in the solid or liquid phase.

The cathode structure includes an electronically conductive component(e.g., a porous metal or alloy), an ionically conductive component(e.g., an electrolyte), and a fluid oxidant (e.g., air, water or aperoxide or other aqueous solution). Advantageously, the cathodestructure may include fluid oxidants that are obtained from the fuelcell's operating environment, such as air or fresh or salt water.

Furthermore, in some embodiments, the active metal fuel cell can becoupled with a PEM H₂/O₂ fuel cell to capture and use the hydrogenreleased, and further improve the energy density and fuel efficiency ofthe system.

These and other features of the invention are further described andexemplified in the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of an active metal anode structureincorporating an ionically conductive protective laminate compositemembrane in accordance with the present invention.

FIG. 1B is a schematic illustration of an active metal anode structureincorporating an ionically conductive protective graded compositemembrane in accordance with the present invention.

FIG. 2 illustrates a solid phase anode embodiment of an active metalfuel cell in accordance with the present invention.

FIG. 3 illustrates a liquid phase anode embodiment of an active metalfuel cell in accordance with the present invention.

FIG. 4 illustrates a Li/water fuel cell and hydrogen generator for a PEMfuel cell in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to specific embodiments of theinvention. Examples of the specific embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to such specific embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

When used in combination with “comprising,” “a method comprising,” “adevice comprising” or similar language in this specification and theappended claims, the singular forms “a,” “an,” and “the” include pluralreference unless the context clearly dictates otherwise. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as commonly understood to one of ordinary skill in the art towhich this invention belongs.

Introduction

The present invention provides an active metal fuel cell. The fuel cellhas a renewable active metal (e.g., lithium) anode and a cathodestructure that includes an electronically conductive component (e.g., aporous metal or alloy), an ionically conductive component (e.g., anelectrolyte), and a fluid oxidant (e.g., air, water or a peroxide orother aqueous solution). The pairing of an active metal anode with acathode oxidant in a fuel cell is enabled by an ionically conductiveprotective membrane on the surface of the anode facing the cathode. Themembrane is composed of one or more materials configured to provide afirst surface chemically compatible with the active metal of the anodein contact with the anode, and a second surface substantially imperviousto and chemically compatible with the cathode structure and in contactwith the cathode structure.

The active metal anode is renewable in that it is configured forreplacement or supplementation of the active metal to provide a fuelsupply for continuous operation of the fuel cell for as long as desired.For example, prior to or during operation of the fuel cell, additionallithium, for example, may be added to the anode by contacting theexisting lithium of the anode with additional lithium having a bond coatsuch as a thin layer of Ag, Al, Sn or other suitable Li alloy-formingmetal in an inert environment. The new Li/Ag alloys to the old therebysupplementing it or “replacing” it as it is depleted in the fuel cellredox reaction with the cathode oxidant. Alternatively, the active metalfuel of the anode could be continuously supplied to the membrane byvirtue of it being dissolved in a suitable solvent, such as, in the caseof lithium, hexamethyl phosphoramide (HMPA), ammonia, organic amides,amines, or other suitable solvents.

The cathode structure includes an electronically conductive component(e.g., a porous metal or alloy), an ionically conductive component(e.g., an electrolyte), and a fluid oxidant in the gas or liquid state(e.g., air, water or a peroxide, such as hydrogen peroxide, or otheraqueous solution). Like the fuel of the anode, the oxidant of thecathode may be continuously supplemented and the waste products removedby flushing fresh oxidant and optionally electrolyte through the cathodestructure.

Furthermore, in some embodiments, the active metal fuel cell can becoupled with a PEM H₂/O₂ fuel cell to capture and use the hydrogenreleased, and further improve the energy density and fuel efficiency ofthe system.

Protective Membranes

The present invention concerns alkali (or other active) metal fuel cellsand electrochemical cells incorporating them. The fuel cell fuelelectrode (anode) has a highly ionically conductive (at least about10⁻⁵S/cm to 10⁻⁴S/cm, and as high as 10⁻³S/cm or higher) protectivemembrane adjacent to the alkali metal electrode that effectivelyisolates (de-couples) the alkali metal electrode from solvent,electrolyte processing and/or cathode environments, including suchenvironments that are normally highly corrosive to Li or other activemetals, and at the same time allows ion transport in and out of thesepotentially corrosive environments. The protective membrane is thuschemically compatible with active metal (e.g., lithium) on one side anda wide array of materials, including those including those that arenormally highly corrosive to Li or corrosive to Li or other activemetals on the other side, while at the same time allowing ion transportfrom one side to the other. In this way, a great degree of flexibilityis permitted the other components of an electrochemical device, such asa fuel cell, made with the protected active metal electrodes. Isolationof the anode from other components of a fuel cell or otherelectrochemical cell in this way allows the use of virtually anysolvent, electrolyte and/or cathode material in conjunction with theanode. Also, optimization of electrolytes or cathode-side solventsystems may be done without impacting anode stability or performance.

In a specific embodiment, the protective membrane is composed of atleast two components of different materials having different chemicalcompatibility requirements. By “chemical compatibility” (or “chemicallycompatible”) it is meant that the referenced material does not react toform a product that is deleterious to fuel cell operation when contactedwith one or more other referenced fuel cell components or manufacturing,handling or storage conditions.

A first material component of the composite is ionically conductive, andchemically compatible with an active metal electrode material. Chemicalcompatibility in this aspect of the invention refers both to a materialthat is chemically stable and therefore substantially unreactive whencontacted with an active metal electrode material. It may also refer toa material that is chemically stable with air, to facilitate storage andhandling, and reactive when contacted with an active metal electrodematerial to produce a product that is chemically stable against theactive metal electrode material and has the desirable ionic conductivity(i.e., a first component material). Such a reactive material issometimes referred to as a “precursor” material.

A second material component of the composite is substantiallyimpervious, ionically conductive and chemically compatible with thefirst material component and the environment of the cathode paired withthe anode. In the case of a fuel cell, the cathode environment is acathode structure that includes an electronically conductive component(e.g., a porous metal or alloy), an ionically conductive component(e.g., an electrolyte), and a fluid oxidant (e.g., air, water or aperoxide or other aqueous solution). By substantially impervious it ismeant that the material provides a sufficient barrier to aqueouselectrolytes and solvents and other fuel cell component materials thatwould be materials that would be damaging to the active metal anodematerial to prevent any such damage that would degrade anode performancefrom occurring. Thus, it should be non-swellable and free of pores,defects, and any pathways allowing air, moisture, electrolyte, etc. topenetrate though it to the first material. Additional components arepossible to achieve these aims, or otherwise enhance electrode stabilityor performance. All components of the composite have high ionicconductivity, at least 10⁻⁷S/cm, generally at least 10⁻⁶S/cm, forexample at least 10⁻⁵S/cm to 10⁻⁴S/cm, and as high as 10⁻³S/cm or higherso that the overall ionic conductivity of the multi-component protectivestructure is at least 10⁻⁵S/cm and as high as 10⁻³S/cm or higher.

A protective composite in accordance with the present invention may be alaminate of two (or more) layers having different chemicalcompatibility. A wide variety of materials may be used in fabricatingprotective composites in accordance with the present invention,consistent with the principles described above. For example, a firstlayer of a composite laminate, in contact with the active metal, may becomposed, in whole or in part, of active metal nitrides, active metalphosphides, active metal halides or active metal phosphorusoxynitride-based glass. Specific examples include Li₃N, Li₃P, LiI, LiBr,LiCl, LiF and LiPON. These materials may be preformed and contacted withthe active metal electrode, or they may be formed in situ by contactingthe active metal (e.g., lithium) with precursors such as metal nitrides,metal phosphides, metal halides, red phosphorus, iodine, nitrogen orphosphorus containing organics and polymers, and the like. The in situformation of the first layer may result from an incomplete conversion ofthe precursors to their lithiated analog. Nevertheless, such incompleteconversions meet the requirements of a first layer material for aprotective composite in accordance with the present invention and aretherefore within the scope of the invention.

A second layer of the protective composite may be composed of a materialthat is substantially impervious, ionically conductive and chemicallycompatible with the first material or precursor and the cathodestructure, such as glassy or amorphous metal ion conductors, ceramicactive metal ion conductors, and glass-ceramic active metal ionconductors. Such suitable materials are substantially gap-free,non-swellable and are inherently ionically conductive. That is, they donot depend on the presence of a liquid electrolyte or other agent fortheir ionically conductive properties. Glassy or amorphous Glassy oramorphous metal ion conductors, such as a phosphorus-based glass,oxide-based glass, phosphorus-oxynitride-based glass, sulpher-basedglass, oxide/sulfide based glass, selenide based glass, gallium basedglass, germanium-based glass; ceramic active metal ion conductors, suchas lithium beta-alumina, sodium beta-alumina, Li superionic conductor(LISICON), Na superionic conductor (NASICON), and the like; orglass-ceramic active metal ion conductors. Specific examples includeLiPON, Li₃PO₄.Li₂S.SiS₂, Li₂S.GeS₂.Ga₂S₃, Li₂O.11Al₂O₃, Na₂O.11Al₂O₃,(Na, Li)_(1+x)Ti_(2-x)Al_(x)(PO₄)₃ (0.6≦x≦0.9) and crystallographicallyrelated structures, Na₃Zr₂Si₂PO₁₂, Li₃Zr₂Si₂PO₁₂, Na₅ZrP₃O₁₂,Na₅TiP₃O₁₂, Na₃Fe₂P₃O₁₂, Na₄NbP₃O₁₂, Li₅ZrP₃O₁₂, Li₅TiP₃O₁₂, Li₃Fe₂P₃O₁₂and Li₄NbP₃O₁₂, and combinations thereof, optionally sintered or melted,may be used. Suitable ceramic ion active metal ion conductors aredescribed, for example, in U.S. Pat. No. 4,985,317 to Adachi et al.,incorporated by reference herein in its entirety and for all purposes.

A particularly suitable glass-ceramic material for the second layer ofthe protective composite is a lithium ion conductive glass-ceramichaving the following composition:

Composition mol % P₂O₅ 26-55%  SiO₂ 0-15% GeO₂ + TiO₂ 25-50%  in whichGeO₂ 0--50% TiO₂ 0--50% ZrO₂ 0-10% M₂O₃ 0 < 10% Al₂O₃ 0-15% Ga₂O₃ 0-15%Li₂O 3-25%and containing a predominant crystalline phase composed ofLi_(1+x)(M,Al,Ga)_(x)(Ge_(1-y)Ti_(y))_(2-x)(PO₄)₃ where X≦0.8 and0≦Y≦1.0 and where M is an element selected from the group consisting ofNd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb and/or andLi_(1+x+y)Q_(x)Ti_(2-x)Si_(y)P_(3-y)O₁₂ where 0≦X≦0.4 and 0≦Y≦0.6, andwhere Q is Al or Ga. The glass-ceramics are obtained by melting rawmaterials to a melt, casting the melt to a glass and subjecting theglass to a heat treatment. Such materials are available from OHARACorporation, Japan and are further described in U.S. Pat. Nos.5,702,995, 6,030,909, 6,315,881 and 6,485,622, incorporated herein byreference.

Either layer may also include additional components. For instance, asuitable active metal compatible layer (first layer) may include apolymer component to enhance its properties. For example, polymer-iodinecomplexes like poly(2-vinylpyridine)-iodine (P2VP-I₂),polyethylene-iodine, or tetraalkylammonium-iodine complexes can reactwith Li to form a LiI-based film having significantly higher ionicconductivity than that for pure LiI. Also, a suitable first layer mayinclude a material used to facilitate its use, for example, the residueof a wetting layer (e.g., Ag) used to prevent reaction between vaporphase lithium (during deposition) and LiPON when LiPON is used as afirst layer material.

In addition, the layers may be formed using a variety of techniques.These include deposition or evaporation (including e-beam evaporation)or thermal spray techniques such as plasma spray of layers of material,such as Li₃N or an ionically conductive glass (e.g., LiPON). Also, asnoted above, the active metal electrode adjacent layer may be formed insitu from the non-deleterious reaction of one or more precursors withthe active metal electrode. For example, a Li₃N layer may be formed on aLi anode by contacting CuN₃ with the Li anode surface, or Li₃P may beformed on a Li anode by contacting red phosphorus with the Li anodesurface.

Such compositions, components and methods for their fabrication aredescribed in U.S. Provisional Patent Application No. 60/418,899, filedOct. 15, 2002, titled IONICALLY CONDUCTIVE COMPOSITES FOR PROTECTION OFANODES AND ELECTROLYTES, its corresponding U.S. patent application Ser.No. 10/686,189, filed Oct. 14, 2003, and titled IONICALLY CONDUCTIVECOMPOSITES FOR PROTECTION OF ACTIVE METAL ANODES, WHICH IS NOW U.S. Pat.No. 7,282,296, ISSUED ON Oct. 16, 2007; US Oct. 16, 2007; U.S. patentapplication Ser. No. 10/731,771, filed Dec. 5, 2003, and titledIONICALLY CONDUCTIVE COMPOSITES FOR PROTECTION OF ACTIVE METAL ANODES,WHICH IS NOW U.S. Pat. No. 7,282,302, ISSUED ON Oct. 16, 2007; and U.S.patent application Ser. No. 10/772,228, filed Feb. 3, 2004, and titledIONICALLY CONDUCTIVE MEMBRANES FOR PROTECTION OF ACTIVE METAL ANODES ANDBATTERY CELLS, WHICH IS NOW U.S. Pat. No. 7,390,591, ISSUED ON Jun. 24,2008. These applications are incorporated by reference herein in theirentirety for all purposes.

FIG. 1A illustrates an anode structure incorporating a protectivecomposite membrane in accordance with one embodiment of the presentinvention. The structure 100 includes an active metal electrode 108,e.g., lithium, bonded with a protective composite laminate 102. Theprotective composite laminate 102 is composed of a first layer 104 of amaterial that is both ionically conductive and chemically compatiblewith an active metal electrode material. The first layer, in contactwith the active metal, may be composed, in whole or in part, of activemetal nitrides, active metal phosphides, active metal halides or activemetal phosphorus oxynitride-based glasses. Specific examples includeLi₃N, Li₃P, LiI, LiBr, LiCl and LiF. In at least one instance, LiPON,the first material is chemically compatible with oxidizing materials.The thickness of the first material layer is preferably about 0.1 to 5microns, or 0.2 to 1 micron, for example about 0.25 micron. Anotherpossibility would be the use of Li₅La₃M₂O₁₂ which is claimed to bestable to molten lithium and have an ionic conductivity of about 10⁻⁶S/cm¹.

These first layer materials may be contacted with the active metal, orthey may be formed in situ by contacting lithium (or other active metal)with precursors such as metal nitrides, metal phosphides, metal halides,red phosphorus, iodine and the like. The in situ formation of the firstlayer may be by way of conversion of the precursors to a lithiatedanalog, for example, according to reactions of the following type (usingP, CuN₃, and PbI₂ precursors as examples):3Li+P═Li₃P (reaction of the precursor to form Li-ion conductor);  1.3Li+Cu₃N═Li₃N+3Cu (reaction to form Li-ion conductor/metalcomposite);  2(a).2Li+PbI₂═2LiI+Pb (reaction to form Li-ion conductor/metalcomposite).  2(b).

First layer composites, which may include electronically conductivemetal particles, formed as a result of in situ conversions meet therequirements of a first layer material for a protective composite inaccordance with the present invention and are therefore within the scopeof the invention.

A second layer 106 of the protective composite is composed of asubstantially impervious, ionically conductive and chemically compatiblewith the first material or precursor, including glassy or amorphousmetal ion conductors, such as a phosphorus-based glass, oxide-basedglass, phosphorus-oxynitride-based glass, sulpher-based glass,oxide/sulfide based glass, selenide based glass, gallium based glass,germanium-based glass; ceramic active metal ion conductors, such aslithium beta-alumina, sodium beta-alumina, Li superionic conductor(LISICON), Na superionic conductor (NASICON), and the like; orglass-ceramic active metal ion conductors. Specific examples includeLiPON, Li₃PO₄.Li₂S.SiS₂, Li₂S.GeS₂.Ga₂S₃, Li₂O.11Al₂O₃, Na₂O.11Al₂O₃,(Na, Li)_(1+x)Ti_(2-x)Al_(x)(PO₄)₃ (0.6≦x≦0.9) and crystallographicallyrelated structures, Na₃Zr₂Si₂PO₁₂, Li₃Zr₂Si₂PO₁₂, Na₅ZrP₃O₁₂,Na₅TiP₃O₁₂, Na₃Fe₂P₃O₁₂, Na₄NbP₃O₁₂, Li₅ZrP₃O₁₂, Li₅TiP₃O₁₂, Li₃Fe₂P₃O₁₂and Li₄NbP₃O₁₂, and combinations thereof, optionally sintered or melted.Suitable ceramic ion active metal ion conductors are described, forexample, in U.S. Pat. No. 4,985,317 to Adachi et al., incorporated byreference herein in its entirety and for all purposes. Suitableglass-ceramic ion active metal ion conductors are described, forexample, in U.S. Pat. Nos. 5,702,995, 6,030,909, 6,315,881 and6,485,622, previously incorporated herein by reference and are availablefrom OHARA Corporation, Japan.

The ionic conductivity of the composite is at least 10⁻⁶S/cm, generallyat least at least 10⁻⁵S/cm to 10⁻⁴S/cm, and as high as 10⁻³S/cm orhigher. The thickness of the second material layer is preferably about0.1 to 1000 microns, or, where the ionic conductivity of the secondmaterial layer is between about 10⁻⁵ about 10⁻³ S/cm, 10 to 1000microns, preferably between 1 and 500 micron, and more preferablybetween 10 and 100 microns, for example 20 microns.

The layers may be formed using a variety of techniques. These includedeposition or evaporation (including e-beam evaporation) or thermalspray methods such as vacuum plasma spray of layers of material, such asLi₃N or an ionically conductive glass. Also, as noted above, the activemetal electrode adjacent layer may be formed in situ from thenon-deleterious reaction of one or more precursors with the active metalelectrode. For example, a Li₃N layer may be formed on a Li anode bycontacting CuN₃ with the Li anode surface, or Li₃P may be formed on a Lianode by contacting red phosphorus with the Li anode surface.

Also, an approach may be used where a first material and second materialare coated with another material such as a transient and/or wettinglayer. For example, an OHARA glass ceramic plate is coated with a LiPONlayer, followed by a thin silver (Ag) coating. When lithium isevaporated onto this structure, the Ag is converted to Ag—Li anddiffuses, at least in part, into the greater mass of deposited lithium,and a protected lithium electrode is created. The thin Ag coatingprevents the hot (vapor phase) lithium from contacting and adverselyreaction with the LiPON first material layer. After deposition, thesolid phase lithium is stable against the LiPON. A multitude of suchtransient/wetting (e.g., Al, Sn or other Li alloy-forming metal) andfirst layer material combinations can be used to achieve the desiredresult.

In addition to protection of the first layer material against reactionwith Li, a Li alloy-forming metal film can serve two more purposes. Insome cases after formation the first layer material the vacuum needs tobe broken in order to transfer this material through the ambient or dryroom atmosphere to the other chamber for Li deposition. The metal filmcan protect the first layer against reaction with components of thisatmosphere. In addition, the Li alloy-forming metal can serve as abonding layer for reaction bonding of Li to the first layer material.When lithium is deposited onto this structure, the Ag is converted toAg—Li and diffuses, at least in part, into the greater mass of depositedlithium.

In many implementations of the present invention, active metal electrodematerial (e.g., lithium) will be applied to the first layer materialwhich is residing on the second material (the first material having beenpreviously applied to the second material), as described further withreference to specific embodiments below.

In one example, the where LiPON is used as the first material and anOHARA-type glass-ceramic (as described herein) in used as the secondmaterial, the resistivity of LiPON is too large for it to be used in amulti-micron film, but the resistivity of the glass-ceramic is muchlower. Thus, a 20-50 micron film of glass-ceramic protected from a Lielectrode with about a 0.2 micron film of LiPON can be used.

In addition to the protective composite laminates described above, aprotective membrane in accordance with the present invention mayalternatively be a functionally graded layer, as shown in FIG. 1B.Through the use of appropriate deposition technology such as RF sputterdeposition, electron beam deposition, thermal spray deposition, and orplasma spray deposition, it is possible to use multiple sources to laydown a graded film. In this way, the discrete interface between layersof distinct composition and functional character is replaced by agradual transition of from one layer to the other. The result, as withthe discrete layer composites described above, is a bi-functionallycompatible ionically conductive composite 120 stable on one side 114 tolithium or other active metal, and on the other side 116 substantiallyimpervious and stable to the cathode/electrolyte, other battery cellcomponents and preferably to ambient conditions. In this embodiment, theproportion of the first material to the second material in the compositemay vary widely based on ionic conductivity and mechanical strengthissues, for example. In many, but not all, embodiments the secondmaterial will dominate. For example, suitable ratios of first to secondmaterials may be 1-1000 or 1-500, for example about 1 to 200 where thesecond material has greater strength and ionic conductivity than thefirst (e.g., 2000 Å of LiPON and 20-30 microns of OHARA glass-ceramic).The transition between materials may occur over any (e.g., relativelyshort, long or intermediate) distance in the composite. To form aprotected anode, lithium is then bonded to the graded membrane on thefirst component material (stable to active metal) side of the gradedcomposite protective layer, for example as described in U.S. patentapplication Ser. No. 10/686,189, filed Oct. 14, 2003, and titledIONICALLY CONDUCTIVE COMPOSITES FOR PROTECTION OF ACTIVE METAL ANODES,which is now U.S. Pat. No. 7,282,296, issued on Oct. 16, 2007; U.S.patent application Ser. No. 10/731,771, filed Dec. 5, 2003, and titledIONICALLY CONDUCTIVE COMPOSITES FOR PROTECTION OF ACTIVE METAL ANODES,which is now U.S. Pat. No. 7,282,302, issued on Oct. 16, 2007; and U.S.patent application Ser. No. 10/772,228, filed and U.S. patentapplication Ser. No. 10/772,228, filed Feb. 3, 2004, and titledIONICALLY CONDUCTIVE MEMBRANES FOR PROTECTION OF ACTIVE METAL ANODES ANDBATTERY CELLS, which is now U.S. Pat. No. 7,390,591, issued on Jun. 24,2008, previously incorporated by reference herein.

In other embodiments, it may be possible for the protective membrane tobe composed of a single material that is chemically compatible with boththe active metal electrode and any solvent, electrolyte, and/or cathodeenvironments, including such environments that are normally highlycorrosive to active metals, and at the same time allows efficient iontransport from one side of the membrane to the other to the other at ahigh level, generally having ionic conductivity, at least 10⁻⁵S/cm to10⁻⁴S/cm, and as high as 10⁻³S/cm or higher.

Fuel Cell Designs

The protected active metal electrodes described herein enable theconstruction of novel active metal fuel cells. As noted above, activemetals are highly reactive in ambient conditions and can benefit from abarrier layer when used as electrodes. They are generally alkali metalssuch (e.g., lithium, sodium or potassium), alkaline earth metals (e.g.,calcium or magnesium), and/or certain transitional metals (e.g., zinc),and/or alloys of two or more of these. The following active metals maybe used: alkali metals (e.g., Li, Na, K), alkaline earth metals (e.g.,Ca, Mg, Ba), or binary or ternary alkali metal alloys with Ca, Mg, Sn,Ag, Zn, Bi, Al, Cd, Ga, In. Preferred alloys include lithium aluminumalloys, lithium silicon alloys, lithium tin alloys, lithium silveralloys, and sodium lead alloys (e.g., Na₄Pb). A preferred active metalfuel electrode (anode) is composed of lithium.

One example of such a fuel cell in accordance with the present inventionis a lithium fuel cell, as illustrated in FIG. 2. The fuel cell includesan lithium fuel electrode (anode) in the solid state. Alternatively,another active metal, particularly an alkali metal, may be used. Thelithium metal electrode can be bonded to a lithium ion conductivemembrane according to any of the techniques described herein and in theapplications incorporated by reference, as described above, with orwithout the use of a bond coat such as a thin layer of Ag, Al, Sn orother suitable Li alloy-forming metal, depending upon the techniqueused. The cell also includes a cathode structure having anelectronically conductive component, an ionically conductive component,and a fluid oxidant.

The cathode structure's electronically conductive component is providedadjacent to the protective membrane on the anode and provides electrontransport from the anode (via a cathode current collector) andfacilitates electroreduction of the cathode oxidant. It may be, forexample, a porous metal or alloy, such as porous nickel. The ionicallyconductive component is generally a fluid electrolyte, and preferably anaqueous electrolyte, for example salt water, or aqueous solutions ofLiCl, LiBr, LiI, LiOH, NH₄Cl, NH₄Br, or other suitable electrolytesalts. The fluid oxidant may be air, water or a peroxide or otheraqueous solution.

As noted above, in some embodiments, the electronically conductivecomponent may be composed of porous nickel. Still further, theelectronically conductive component may be treated with an ionomer, suchas per-fluoro-sulfonic acid polymer film (e.g., du Pont NAFION) toexpand the range of acceptable electrolytes to those having little or nonative ionic conductivity. An additional advantage of ionomers likeNAFION is that the salt is chemically bonded to the polymer backbone,and therefore cannot be flushed out, so if a liquid oxidant such ashydrogen peroxide were to flow through the cathode, it would not benecessary to flush the prior electrolyte salt out of the cathode toavoid having salt dissolved in the peroxide solution.

An example of a suitable cathode structure is an air electrodeconventionally used in metal (e.g., Zn)/air batteries or low temperature(e.g., PEM) fuel cells.

As the fuel cell operates to generate electricity, the lithium metal ofthe renewable anode is consumed. The metal is then supplemented withfresh lithium metal, as required, to provide continuous operation for aslong as desired. For example, prior to or during operation of the fuelcell, additional lithium may be added to the anode by contacting theexisting lithium of the anode with additional lithium having a bondcoat, such as a thin layer of Ag or other suitable alloying metal, in aninert environment. The Ag layer reacts with the surface of the existingLi forming Li—Ag alloy. The Li—Al alloy layer serves as a strongreaction bond between the additional Li and the existing lithium. Thenew Li/Ag alloys to the old thereby supplementing it or “replacing” itas it is depleted in the fuel cell redox reaction with the cathodeoxidant. In this way, the renewable lithium anode can be replaced orsupplemented through the use of a thin bonding foil such as Ag, Al, orSn foil, as shown in the figure, as it is depleted.

Like the fuel of the anode, the oxidant of the cathode may becontinuously supplemented and the waste products removed by flushingfresh oxidant and optionally electrolyte through the cathode structure.The cathode oxidant can thus be continuously supplied with oxygen fromeither air or water or from a liquid oxidant such as peroxide. The cellthen operates as a fuel cell where the Li⁺ conductive membrane andelectronically conductive component of the cathode structure are static,and the Li anode material is continuously replaced as it is depleted, asis the cathode oxidant (e.g., air, water or peroxide) on the other sideof the protective membrane.

In another embodiment, depicted in FIG. 3, the lithium (or other activemetal) anode could be continuously supplied to the membrane by virtue ofit being dissolved in a suitable solvent. For lithium, suitable solventsinclude hexamethyl phosphoramide (HMPA), liquid ammonia, organic amides,amines, in particular methylamine, and mixtures thereof, and othersuitable solvents. Lithium is known to dissolve in HMPA in highconcentration to form stable solutions of solvated electrons. In thisway, bulk lithium metal can be fed into a constant volume of HMPA,keeping the Li/HMPA solution near or at the solubility limit. As lithiumis transported across the protective membrane more lithium metal willdissolve into the HMPA solution, and the cell acts as a true Li/air orLi/water fuel cell. Thus, in this embodiment, lithium metal iscontinuously supplied to the Li/HMPA solution anode, while air or water(cathode oxidant) is supplied to the cathode structure. The energydensity of such a device will be very high since the weight of thepassive components are negligible relative to the fuel and air supply.

In a fuel cell, any part of the active metal electrode that is notcovered by the protective membrane will generally be sealed off from thecorrosive environments, such as by a current collector material (e.g.,copper), an o-ring seal, a crimp seal, polymer or epoxy sealant, orcombination of these.

In addition, by coupling the Li/water fuel cell as described herein witha PEM H₂/O₂ fuel cell, as illustrated in FIG. 4, hydrogen released fromthe Li/water fuel cell can be captured and the energy density and fuelefficiency of this system may be further further increased.

Conclusion

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theinvention. It should be noted that there are many alternative ways ofimplementing both the process and compositions of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

All references cited herein are incorporated by reference for allpurposes.

1. An alkali metal fuel cell, comprising: an anode comprising solidalkali metal as fuel; a cathode structure comprising a staticelectronically conductive component, an ionically conductive componentcomprising an electrolyte for ions of the alkali metal, and a fluidoxidant obtained from an operating environment of the cell; a protectivemembrane interposed between the anode and the cathode structure, themembrane conductive to ions of the alkali metal, the membrane isolatingthe anode from the fluid oxidant of the cathode structure; and alkalimetal dissolved in a solvent on the anode side of the membrane.
 2. Thecell of claim 1, wherein the fluid oxidant comprises a liquid oxidant.3. The cell of claim 2, wherein the liquid oxidant comprises an aqueoussolution.
 4. The cell of claim 1, wherein the electrolyte compriseswater.
 5. The cell of claim 1, wherein the electrolyte comprises anaqueous solution.
 6. The cell of claim 1, wherein the electrolyte isflushed through the cathode structure.
 7. The cell of claim 1, whereinthe alkali metal is lithium or a lithium alloy.
 8. A method of providingelectrical power, the method comprising: i) discharging an alkali metalfuel cell to provide electrical power, the cell comprising, an anodecomprising solid alkali metal as fuel, a cathode structure comprising astatic electronically conductive component, an ionically conductivecomponent comprising an electrolyte for ions of the alkali metal, and afluid oxidant obtained from an operating environment of the cell, aprotective membrane interposed between the anode and the cathodestructure, the membrane conductive to ions of the alkali metal, themembrane isolating the anode from the fluid oxidant of the cathodestructure, and alkali metal dissolved in a solvent on the anode side ofthe membrane; and ii) adding fresh solid alkali metal fuel for the anodesuch that discharge of the cell continues.
 9. The method of claim 8,further comprising repeating the discharging and adding until celloperation is no longer desired.
 10. The method of claim 8, furthercomprising supplementarily replacing the electrolyte.
 11. The method ofclaim 10, wherein the supplementarily replacing the electrolyte includesflushing the cathode structure with electrolyte.
 12. The method of claim8, wherein the fluid oxidant further comprises a liquid oxidant, and themethod comprising: i) discharging the fuel cell; ii) supplementarilyadding fresh solid alkali metal fuel for the anode; iii) supplementarilyreplacing the liquid oxidant; and iv) repeating the discharging, addingand replacing until cell operation is no longer desired.
 13. The methodof claim 12, wherein the supplementarily replacing the liquid oxidantincludes flushing the cathode structure with liquid oxidant.
 14. Themethod of claim 13, wherein the liquid oxidant comprises water.
 15. Themethod of claim 13, wherein the liquid oxidant comprises an aqueoussolution.
 16. A method of removing waste product from an alkali metalfuel cell, the method comprising: i) discharging an alkali metal fuelcell, the cell comprising, an anode comprising solid alkali metal asfuel, a cathode structure comprising a static electronically conductivecomponent, an ionically conductive component comprising an electrolytefor ions of the alkali metal, and a fluid oxidant obtained from anoperating environment of the cell, a protective membrane interposedbetween the anode and the cathode structure, the membrane conductive toions of the alkali metal, the membrane isolating the anode from thefluid oxidant of the cathode structure, and alkali metal dissolved in asolvent on the anode side of the membrane, thereby creating wasteproducts in the cathode structure; and ii) flushing the cathodestructure to remove the waste products from the cathode structure. 17.The method of claim 16, wherein the flushing comprises flushingelectrolyte through the cathode structure.
 18. The method of claim 16,wherein the flushing comprises flushing liquid oxidant through thecathode structure.
 19. The method of claim 18, wherein the liquidoxidant comprises water.
 20. The method of claim 16, further comprisingrepeating the discharging and flushing until cell operation is no longerdesired.